Download Chapter 22 Thalamus

Chapter 22; Fundamentals of Sensory Systems
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11/21/07
Each system responds with some specificity to a stimulus and each employs
specialized cells—peripheral receptors—to translate the stimulus into a signal
that all neurons can use
Labor is divided among neurons such that not only different stimulus energies but
also different stimulus qualities are analyzed by separate groups of neurons
Sensory Systems (SS) provide the CNS w/ a representation of the external world
 Sensory Systems perform common types of operations
1. Ability of each system to compare events that occur simultaneously at
different receptors
a. Brings about the greatest response where the difference in stimulus
strength is greatest
2. Systems make comparisons w/ past events and w/ sensations received by other
sensory systems
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Sensory systems provide the CNS w/ a constantly updated representation of the
external world
Requires close interaction between ascending pathways and descending
mechanisms
o Ascending (Bottom-Up) = stimulus-driven; peripheral receptors
o Descending Mechanisms; sort out from the large amount of sensory input
events that need immediate attention, alter ascending inputs in ways that
optimize perception
Perception of a sensory experience can change even though the input remains the
same
Detection/ Recognition of a stimulus is referred to as __________
Interpretation and appreciation of that event constitutes ____________.
Psychophysics
 Psychophysical experiment determines the quantitative relationship b/w a
stimulus and a sensation; used to establish the limits of sensory performance
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Receptors; specific for a narrow range of input
 Receptors; specialized cells which convert external stimulus energy into a signal
that produces a neural response
 Each of the fundamental types of stimuli has a separate population of receptors
selective for the particular form of energy
 Within a single sensory system are classes of receptors that are particularly
sensitive to one stimulus and not another (i.e. hot or cold)
 These different functions individual receptors serve is directly associated to
receptor structure and chemistry
 Receptors transduce energy into a change in membrane voltage
 Transmit voltage change to ganglion cells that send their axons into CNS
Receptor types vary across sensory systems
 Correlation between the number of receptor types within a system and the types
of stimuli that system is able to detect
o Somatosensory; large number of receptor types and ability to detect many
different types of stimuli
o Auditory; two classes of receptor—inner hair cells and out hair cells of
the cochlea—transduce mechanical energy of the basilar membrane
o Visual system; transduction performed by rods and cones
Receptors have a characteristic pattern of position and density
 Orderly arrangement of receptors exists along skin, basilar membrane, and retina
 Differences in peripheral innervation density are tightly correlated w/ spatial
acuity
Receptors; sites of convergence and divergence
 A single ganglion cell receives input from several receptors and in many cases a
single receptor sends information to two or more ganglion cells.
 Convergence and divergence go hand-in-hand for the Somatosensory system
o An individual receptor is often innervated by axons of several ganglion
cells while at the same time branching its projections to become part of
several receptor organs
 If spatial resolution is a requirement; convergence of receptor inputs is low
 For detection of weak signals; convergence of receptor inputs is high
 If a receptors input is used for a complex function; divergence of input from
receptor onto many ganglion cells occurs
Receptors vary in embryonic origin
 Auditory and Somatosensory system receptors and ganglion cells are part of PNS
and therefore originate from neural crests
 Retina is generated from embryonic diencephalons; part of the CNS and therefore
originates from neural tube
Sensory information is transmitted along labeled lines
 Labeled Line Principle; when a particular population of neurons is active the
conscious perception is of a specific stimulus
 Receptors are selective not only in what drives them but also in the postsynaptic
targets with which they communicate
 Orderly relay from receptor to ganglion cell to CNS makes up labeled line
 Modality: all sensory information arising from a single class of receptors
 Labeled lines means that neurons in sensory systems carry specific modalities
 Topographic Pattern; neurons positioned side by side in one region
communicate w/ neurons so positioned in the next region
Neural signaling is by a combination of rate and temporal codes
 The firing rate or frequency of A.P.s signals the strength of the sensory input
 Temporal code in some systems
Lateral Mechanisms enhance sensitivity to contrast
 Lateral Inhibition; sensory neuron displays a receptive field with an excitatory
center and an inhibitory surround
o Enhances contrast, sharpens the selectivity of a neuron
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Thalamus
Axons from each sensory system cross the midline on their way to the thalamus
 Decussate-tendency for axons to cross the midline on way to thalamus
 Serve the broad function of bringing the axons together carrying sensory
information into alignment with motor output
Specific Thalamic Nuclei Exist for each sensory system
 Information from each sensory system must be relayed through the thalamus on
its way to the cerebral cortex
 Neurons performing one function are segregated from those performing another
 For each thalamic nucleus; Population of large and small neurons
o Population of large neurons; carry the most rapidly transmitted signals
from the periphery to the cortex
Multiple Maps and parallel pathways
 Nuclei in the central pathways often contain multiple maps
 LGN receives direct input from the retina
 Inputs from ganglion cells vary in location, structure, and function
Sensory cortex includes Primary and Association areas
 Primary sensory areas; parts of cortex that receive axons of sensory relay
 Association areas; neighboring areas with which the primary areas communicate
directly
Response mappings and plasticity
 Each area of sensory cortex shares w/ its subcortical components a map of at least
part of the sensory periphery
 Mappings of sensory cortex are plastic
 Neurons in areas of the sensory cortex are organized into six layers
 Middle Layers (III and IV) are main site of termination of axons from thalamus
Columnar Organization
 Columns (neurons stacked upon each other) are similar but differ from neurons on
either side
o I.e. ocular dominance columns, orientation columns, direction selective
columns